Automobiles with automatic transmissions are typically equipped with a fluid coupling device, i.e., a torque converter, between the engine and the transmission. The torque converter transfers torque from the engine to the transmission. The torque converter allows the load from the engine to slip at lower engine operating speeds, such as when the engine is at idle, thereby preventing the engine from stalling. When the engine is operating at higher speeds, the torque converter transfers the torque from the engine to the transmission with little slippage.
A common internal feature of the modern torque converter is an electronically controlled converter clutch, hereinafter referred to as a lock-up clutch. When fully engaged after a predetermined engine load, vehicle speed, and transmission gear ratio is achieved, the lock-up clutch mechanically connects the engine crankshaft to the transmission input shaft, thereby eliminating the speed differential, or slip, inherent to torque converter operation. The result is improved fuel efficiency.
To minimize the transfer of firing pulses from the engine into the transmission when the lock-up clutch is fully engaged, a spring damper is commonly provided in the torque path within the torque converter housing. However, the damping efficiency of spring dampers is typically not uniform across the full vehicle driving range. When operating within ranges of damper inefficiency, such as lower vehicle speeds, it may be desirable to introduce a controlled amount of slip across the lock-up clutch to help reduce the transfer of engine firing pulses into the transmission.
Conventional torque converters including the lock-up clutch are typically provided with two oil flow paths, an apply passage and a release passage. Because the oil in the torque converter is performing work during periods of slip operation, as when the lock-up clutch is disengaged or operating in a controlled slip condition, heat is generated in the oil that must be dissipated. Consequently, the apply and release passages also double as a flow path for circulating cooling oil into and out of the torque converter. Oil exiting the torque converter is typically directed to a heat exchanger where the excess heat is removed.
To apply the lock-up clutch, hydraulic oil is directed into the apply passage, which exerts a hydraulic force on a piston or disk within the torque converter. The piston engages the lock-up clutch and forces it into contact with a reaction member. The reaction member is typically, but not necessarily, a portion of the torque converter housing. Because a portion of the cooling oil flow path is through the gap between the lock-up clutch and the reaction member, this operating mode substantially blocks the flow path through the torque converter, thereby negatively affecting the flow of cooling oil through the torque converter.
To disengage the lock-up clutch, the direction of oil flow is reversed, i.e., the hydraulic oil is directed into the release passage. The hydraulic force acts on the piston in the opposite direction, moving the lock-up clutch away from the reaction member, thereby disengaging it. This mode of operation unblocks the flow path so that the flow of cooling oil may then be restored.
A dedicated spool valve is required to control and/or change the direction of flow into the apply passage or the release passage, to engage and/or disengage the lock-up clutch.
In a torque converter with two oil flow paths as described above, accurately controlling the amount of slip across the lock-up clutch, while simultaneously maintaining a flow of cooling oil into and out of the torque converter, is difficult to accomplish.